The capability to modify properties of waveguide modes in optical waveguides is a fundamental prerequisite for making optical waveguide devices for many applications areas of integrated optics, photonics, and optoelectronics. One such area is the coupling of light between compact planar waveguides and the outside macroscopic world. A low efficiency of this coupling is a major practical problem in the design and fabrication of integrated microphotonic devices. Various proposals have been made to address this problem, but the coupling still remains a challenge particularly for waveguides of sub-micrometer dimensions made in high index contrast (HIC) materials such as III-V semiconductors, silicon oxynitride, and silicon-on-insulator (SOI). Very compact planar waveguide devices can be made in these materials. In SOI waveguides, light is highly confined in the silicon core which can have cross-sections on the order of 200 nm×200 nm or less, and bending radii can be reduced to a few micrometers. Beside the potential for chip size reduction, the benefit of integration of the mainstream microelectronic technology with photonics has been the main driving force in the emerging field of silicon photonics with significant recent improvements in fabrication technology and many novel structures and devices reported, including modulators, lasers, and arrayed waveguide gratings (AWGs).
Due to the large mode effective index and mode size disparities, the optical coupling between an optical fiber and a high index contrast waveguide with a small cross-section is largely inefficient. In order to match a large optical fiber mode to a HIC waveguide mode with an area typically two orders of magnitude smaller, in plane and out-of-plane mode size transforming structures need to be used.
Various techniques are known for mode manipulation in planar waveguide devices. Mode size transforming structures in both the in-plane and out-of-plane directions are conceptually simple, but the out-of-plane tapering requires complex fabrication techniques such as gray-scale lithography, which is not yet a standard technique in the industry. Grating couplers [G. A. Masanovic et al., Dual grating-assisted directional coupling between fibers and thin semiconductor waveguides, IEEE Photon. Technol. Lett. 15, 1395, 2003] have been demonstrated, but their fabrication is demanding, and polarization and wavelength sensitivity is typically large. An interesting approach is to use an inversely tapered waveguide that adiabatically narrows down to a width of about 100 nm or less as the waveguide approaches the facet facing the fiber [V. R. Almeida et al., Nanotaper for compact mode conversion, Opt. Lett. 28, 1302, 2003]. The waveguide effective index is reduced by narrowing the waveguide width, which causes the mode to expand and to eventually match that of the fiber. However, drawbacks of this technique are problems with fabrication reproducibility of the thin taper tip and polarization dependent loss (PDL). As well, this method is mainly suitable for channel waveguides of sub-micrometer size. An alternative approach is to use a coupler with a planar graded-index (GRIN) lens [A. Delage et al., Monolithically integrated asymmetric graded and step-index couplers for microphotonic waveguides, Optics Express 14, 148, 2006]. The structure acts as an asymmetric GRIN lens that is the planar analogue of the conventional cylindrical GRIN lens. The GRIN coupler can be made very compact, about 15 μm in length. However, a reproducible growth of thick GRIN layers requires a material growth development that may add to the fabrication complexity and device cost.
Other forms of mode transformers have also been proposed. Long-period grating couplers have been demonstrated [Z. Weissman and A. Hardy, 2-D mode tapering via tapered channel waveguide segmentation, Electron. Lett. 28, 1514, 1992] for low index contrast waveguides such as those made in a silica-on-silicon platform, but their application in HIC waveguides is hindered by the reflection and diffraction losses incurred at the boundaries of different segments. Such couplers are also comparatively large, i.e. a few hundred micrometer long. To reduce the reflection loss, a non-periodic irregular lateral tapering has been proposed [M. M. Spühler et al., A very short planar silica spot-size converter using a nonperiodic segmented waveguide, J. Lightwave Technol. 16, 1680, 1998]. Still, such mode transformers are quite large (>100 μm in length), the coupling loss reduction is rather modest (˜2 dB) and insufficient for most practical devices.
Thus, it will be appreciated that the ability to manipulate modes in optical waveguides is an essential prerequisite for making integrated waveguide structures and devices. In this invention, a general mechanism is disclosed that can control the waveguide mode propagation in a prescribed manner with little or no detrimental effects such as loss penalty or higher order mode conversion.
A specific example where the need for an efficient mode transformation is essential are junctions between waveguides fabricated from different materials, for example using deposition, growth, or heteroepitaxy, as it is often used when joining together waveguides with different functionalities, for example active (lasers, modulators, photodetectors) and passive waveguide structures. The waveguide effective mode index mismatch at such junctions results in insertion loss and return loss penalties and also in higher order mode excitation. The latter needs to be avoided in devices that rely on single-mode operation, as is the case for most state-of-the-art photonic waveguide devices.
Another important factor affecting the coupling of waveguides to the outside world is the reflectivity of the waveguide facets. Facets are typically formed either by etching or by cleaving with or without a successive polishing step. The reflectivity of the thus fabricated facet is determined by the materials that comprise the waveguide and by the waveguide geometry. Very often, however, there is a need to be able to control the reflectivity of the facets in order to achieve certain device functionalities or to improve device performance. A typical example is the need for low or high reflectivity facets for distributed feedback lasers, optical amplifiers or external cavity semiconductor lasers.
Currently, changing the reflectivity of waveguide facets is done by coating the facet with a single layer or a multilayer of dielectric or metallic films. This process has to be performed at the chip level after the actual formation of the facets by the cleaving or etching process. If a facet with high reflectivity is required for a device this can only be achieved by the deposition of metals or complex multilayer structures comprised of different materials. In addition to the complexity of fabrication these coatings can also introduce additional thermal and mechanical problems to devices.